In various vacuum devices conventionally used for semiconductor manufacturing such as sputtering devices, CVD (Chemical Vapor Deposition) devices and dry etching devices, high temperature active chemical species generated through reactions in such a device stick to the low temperature pipe walls of the vacuum device without being captured onto the semiconductor circuit substrate, then they become dust and scattered, and sticking of this dust to the semiconductor circuit substrate brings a drop in yield. The dust is discharged from the vacuum device, then cooled by the inner wall of its exhaust pipe and sticks and accumulates thereon, causing a phenomenon that the effective bore of the exhaust pipe gradually narrows.
When the effective bore of the exhaust pipe narrows in this way, the piping resistance to the exhaust gas becomes too great to continue vacuuming, and the operation of the vacuum device has to be suspended to scrape off the chemical species having clogged the exhaust pipe.
A conventional way of addressing this problem is to increase the capacity for allowing dust accumulation in the exhaust pipe by increasing its bore to somewhere between 150 and 200 mm and thereby to reduce the required frequency of cleaning.
However, this way of widening the bore of the exhaust pipe makes the capacity of the exhaust pipe greater than that of the vacuum device body, and discharging through the exhaust pipe requires enlarged exhaust capacity of the vacuum pump, entailing a big problem in energy efficiency.
The chemical species accumulating in the exhaust pipe, which are highly reactive with oxygen, would cause an abrupt combustion when coming into contact with air, often causing a fire. To avoid this risk, a conventional exhaust pipe, if deposits build up within, will have to be covered with a plastic sheet, for instance, all over the outside of its exhaust pipe, the exhaust piping being removed from the vacuum device body while letting inert gas flow within the plastic sheet and taken out in that state to scrape off the deposits by shearing. This causes a serious problem of hazardous work that must be done by a number of human actions.
Also, this clogging of the exhaust pipe has been a major bottleneck to the further development of the semiconductor industry as it not only brings down the utilization rate of the equipment but also requires a great amount of cleaning labor.
In view of these problems, the present inventors pursued research to solve the problems attributable to the adherence and accumulation of active chemical species in the exhaust pipes of such vacuum devices, and first discovered that unused chemical species could be prevented from sticking to the exhaust pipe by heating the outer wall of the vacuum device and the exhaust system to or above 140° C., preferably to about 150° C.
Then, the bore of the exhaust pipe of the vacuum device was reduced from the conventional 200 mm to 20 mm, and a heating device comprising an electric heater and a conventional insulator was attached for trial heating, leading to the finding that the required thickness of the insulating material for keeping warmth (that is, the thickness needed for keeping the surface temperature of the insulating material at the room temperature) was 100 to 150 mm. As a result, it was found that the external diameter of the exhaust pipe including the insulating material became around 250 to 350 mm, greater than the thickness of the conventional exhaust pipe.
Unlike this, the exhaust pipe of a conventional vacuum device of this kind is usually penetrated through the access floor on which the vacuum device is installed and the structural floor of the clean room (between which the circulating air of the clean room is passed) and reaches a vacuum pump downstairs. It therefore is made correspondingly more difficult to feed the heating device-equipped exhaust pipe which is greater in sectional area than a conventional pipe through the limited space under the floor on which the vacuum device is installed.
Thus it was found that, as the insulating performance of currently used insulating material would make the material too thick to enable the under-floor piping to be readily arranged, a better performing insulating material had to be employed and its required thickness should be reduced to ⅓ or less of glass wool, for instance. For this reason, it is conceivable to use as the insulating material, for instance, polyurethane foam which is superior to glass wool in insulating performance, but polyurethane foam can endure heat of up to 80° C. at most and is unusable where the regularly employed temperature to which it is exposed is 140° C. or above.
On the other hand, another known conventional vacuum insulating material is prepared by, as described in Patent Document 1 listed presently, sticking together composites each comprising an aluminum film formed by vapor deposition over a sheet of a specific resin as the supporting body, disposing a protective layer over it and providing an adhesive layer on the undersurface of the supporting body, the inner space between the composites being vacuumized and filled with silica powder.
However, this vacuum insulating material, intended for use in electrical household appliances such as cooking heaters or warmth keeping heaters, is only about 200 mm×300 mm in size. Vapor deposition is applicable to the formation of such a small film, but a vacuum insulating material of 1 m or more in length or one for use in industrial equipment or piping cannot be formed by vapor deposition.
The reason is that vapor deposition devices are hardly applicable for use with very large sheets of 1 m or more in length, and moreover it is difficult to fill the spaces in so large sheets uniformly with silica powder. In a semiconductor manufacturing apparatus, the vacuum chamber for 300 mm silicon wafers exceeds 600 mm in diameter and 1.8 m in circumferential length, while in a liquid crystal display panel manufacturing apparatus each side of its outer wall exceeds 3 m in length because it works on glass substrates of 2.2 m×2.3 m. The specified length of piping is usually 4 m. A vacuum insulating material to be used for such purposes is required to be longer than 1 m so as to ensure a reasonable level of working efficiency.    Patent Document 1: Japanese Patent Laid-Open No. 2000-310392